The present invention pertains to the field of walk-through metal detectors. Such metal detectors are not new; virtually anyone who has traveled by commercial air recently has seen a walk-through security metal detector in use. Typical walk-through metal detectors, however, are designed with relatively low sensitivity. Keys, watches, jewelry, buttons, zippers, and the like generally do not contain much metal while guns, knives and similar weapons do. To avoid having airline passengers remove jewelry, keys, and clothing, security metal detectors need only be sensitive enough to detect the amounts of metal in weapons. Such metal detectors are not usable to detect minute amounts of metal, such as is contained in integrated circuit chips.
One form of walk-through type metal detector is shown in Haas U.S. Pat. No. 4,357,535. Haas, however, describes an arch that is susceptible to being bumped, producing vibrations which in turn can cause spurious signals in the detector system due to its high sensitivity.
A coil configuration for a walk-through metal detector is shown in Aittoniemi, et al. U.S. Pat. No. 4,605,898 which is intended to minimize the role of target orientation in detection. A balanced coil configuration is described in Goring Kerr Limited European Patent Application Publication No. 0 208 214. Neither Aittoniemi et al. nor Kerr, however, show means to decrease sensitivity through a predetermined section of the passageway.
The use of a high sensitivity metal detector also poses difficulty due to the presence of metal in the environment surrounding the metal detector. For example, there may be metal reinforcing bars in the floor or metal objects or machinery located near the detector.